This invention relates to a circuit arrangement comprising a switched-mode power supply (SMPS), a source for feeding the SMPS and a control circuit for controlling the SMPS, wherein the SMPS supplies the control circuit with energy during operation, and also comprising a starter circuit for applying a supply current to the control circuit when the arrangement is put into operation.
A circuit arrangement for generating a DC voltage from a sinusoidal input voltage is known from DE-PS 36 12 147, which corresponds to U.S. Pat. No. 4,801,887 (Jan. 31, 1989). This input voltage is used to apply energy to a switched-mode power supply and a control circuit activating the power supply. The voltage required for the control circuit is derived by a circuit arrangement during operation, a capacitor in said circuit arrangement being coupled to a first output terminal of the source and being dischargeable via a first diode arrangement and the switched-mode power supply, said circuit arrangement including a series connection which consists of a second diode arrangement which is connected parallel to the first diode arrangement and via which only the charging current of the capacitor flows, and of at least one parallel connection of the control circuit and a smoothing capacitor, which parallel connection is connected to a terminal of the switched-mode power supply.
Customarily, the switched-mode power supply is switched on by the control circuit when the voltage across the control circuit reaches a threshold value. When the complete circuit arrangement is put into operation, therefore, initially the discharging current of the capacitor cannot flow across the switched-mode power supply. Therefore, the prior art circuit arrangement comprises a starter circuit via which at least the discharging current of the capacitor flows until the threshold value is reached. When the switched-mode power supply has been switched on, the discharging current can flow thereacross.
In the circuit arrangement known from DE-PS 36 12 147, i.e. U.S. Pat. No. 4,801,887 the starter circuit comprises a transistor whose collector is connected to the first connection terminal of the source and whose emitter is coupled to the junction of the second diode arrangement and the parallel connection of the control circuit and the smoothing capacitor. The discharging current of the capacitor flows via the collector-emitter path of the transistor and a preceding collector resistance. The transistor as well as the collector resistance, therefore, are to be proportioned for the current whereby the smoothing capacitor connected parallel to the control circuit is to be charged and whereby the control circuit is to be powered upon activation. Moreover, during operation these elements are exposed to at least approximately the source voltage. Especially when the switched-mode power supply is powered from a power supply mains, the transistor and the collector resistance must be proportioned for a high voltage and comparatively large currents. For circuit components which are required to operate for only very brief periods of time in comparison with the overall period of operation of the circuit arrangement, this is uneconomical, especially when they are used in small and inexpensive domestic appliances.